Word for how well something actually works
It's a one word term that means how well something actually works??
Different from something that works well, it's like.. imagine shopping for bras:
Some people buy bras based on looks, but some people don't value looks over {blank}.
(Meaning how well it supports, and all that.)
I think it starts with a c but I could be wrong.
function
noun
the kind of action or activity proper to a person, thing, or institution; the purpose for which something is designed or exists; role.
Dictionary.com
This edit is from AndyT's comment. It gives you another noun to use that more closely matches the requested usage of qualifying whether or not something is working as designed. And from the discussion in the comments, I now prefer functionality over function.
functionality
noun
1 The quality of being suited to serve a purpose well; practicality. ‘I like the feel and functionality of this bakeware’
1.1 The purpose that something is designed or expected to fulfil.
Oxford
This edit also stems from the comments and discussion. It provides different parts of speech to use in sentences that are structured for such.
function
verb (used without object)
- to perform a specified action or activity; work; operate: The computer isn't functioning now. He rarely functions before noon.
- to have or exercise a function; serve: In earlier English the present tense often functioned as a future. This orange crate can function as a chair.
Dictionary.com
functional
adjective
- of or relating to a function or functions : functional difficulties in the administration.
- capable of operating or functioning : When will the ventilating system be functional again?
- having or serving a utilitarian purpose; capable of serving the purpose for which it was designed
Dictionary.com
Common usages of these words in everyday speech:
"This phone is ugly, but I love its functionality."
"How well does this computer function?" "It is old, so not well."
"Your brand new Ford breaks down all the time. I drive an '85 Dodge, but at least it's functional."
Consider efficacy.
efficacy noun The ability to produce a desired or intended result. ‘there is little information on the efficacy of this treatment’ - ODO
Your example would then be:
- Some people buy bras based on looks, but some people don't value looks over efficacy.
You can also use utility, usefulness or usability.
ODO:
utility NOUN
1 The state of being useful, profitable, or beneficial.
‘he had a poor opinion of the utility of book learning’usefulness NOUN
The quality or fact of being useful.
‘faults that affect the book's usefulness’usability (also useability) NOUN
The degree to which something is able or fit to be used.
‘it was important to measure the usability of each product’
One word which does start with a c meaning "how well something can do its job" is capability.
The power or ability to do something.
‘he had an intuitive capability of bringing the best out in people’
‘the company's capability to increase productivity’1.1 (often capabilities) The extent of someone's or something's ability.
— Oxford
A similar word is capacity:
2.1 The ability or power to do something.
— Oxford
However, since the principal use of capacity is to do with volume, how much something can contain or hold, using it in the example sentence may not be felicitous.
How about effectiveness?
From Dictionary.com:
effective - adjective
1. adequate to accomplish a purpose; producing the intended or expected result:'effective teaching methods; effective steps toward peace.'
Related forms: effectiveness, noun
From Oxford:
effectiveness - noun
The degree to which something is successful in producing a desired result; success.
'the effectiveness of the treatment'